General

Tasks

When LOS is initiated via the control panel switches, it creates core partitions for each task defined in the installation’s configuration table, and assigns a unique Task Number (from 1 upwards) to each I/O station and to each Printer. Each task is initiated at the start of the appropriate control program, and thereafter all tasks run asynchronously under the control of the Task Scheduler.

• The I/O station Control Program outputs the PROGRAM? prompt and, upon receipt of a valid reply, overlays the station’s core partition with the requested program and passes control to it. The program must return to the Control Program on completion (or on operator’s ESCAPE request) (by JUMP Z 1400 or JUMP Z 1402), whereupon the PROGRAM? prompt will be re-output.

• The Printer Control Program tests the Print Queue currently assigned to the printer for the presence of work postings. If work is found, the oldest posting is removed from the queue and assigned to the printer. The appropriate program is overlaid into the printer’s core partition unless it is already in the partition, and control is passed to it.
  The program must return to the Control Program upon completion (by JUMP Z 1406 or JUMP Z 1402). If there is no work in the queue, the Control Program waits for postings to the queue and the printer is IDLE. The operators may, by command, change the print queue assignment at any time; the Control Program will switch to the new queue as soon as it receives such instructions.

Inter-Task Communication

I/O Station ® Printer

For any application requiring printout, the I/O station task does not directly output to a printer; rather all output is written to the disc-held Spool File for later processing by a printer task. The format and content of these intermediate Spool records is defined by the application, except that each record is 124 words long (the last 4 words of the sector are reserved for system use: double-word program name and two chain-linking words).

As a general rule, the I/O station task should create compact spool records with the minimum of processing beyond input validation; wherever possible processing and updating should be carried out (at a lower priority level) by the printer task. This improves I/O station response time, reduces the spool storage requirement and eases the problems of recovery after a cancellation request or machine fault.

An application may require several spool records to define a printout (e.g. an invoice may be of any length), but usually a single record will suffice. The first record is referred to as the header record. When spooling is complete, the spooling task posts the header record number to a pre-selected print queue, normally determined by the type of paper required, and the system adds the posting to the end of the queue.

The system inserts the station’s task number into the first word of the Spool Buffer when an I/O station starts a new program, and after each SPOOL and each POST subroutine call (thus the receiving printer program may identify the station from which it has received work). Note: this provision does not reserve the first word of spool records.

Printer ® I/O Station

The FLASH SINGLE STATION subroutine allows any task to send a message to a specific I/O Station. A printer may identify the Station from which it has received work by the task number in the first word of the spool buffer. If this task number is stored at 3742- (within the Printer’s Task Control Area) the system will flash the station pppp PRINTED BY x when the program returns to the control program (by JUMP Z 1406).

System ® Operator

The general FLASH subroutine allows any task to send a message to all I/O Stations.

Operator ® System

All I/O Stations are provided with a key which interrupts the current program and allows the operator to enter system commands. Available commands are listed in section 3 of this guide.

Operator ® Operator

Program S is available to send a message to the screen of any or all I/O Stations.

Partition Map (2k Version)

Each task has its own core partition. All task partitions are of the same size and format (as defined in the Configuration Table). Because the partition maps are pre-defined and known throughout the system, there is a considerable saving in the number of parameters required by subroutines. Experience has shown a 2K partition size to be the optimum choice for Commercial applications, and all offsets quoted in this guide will refer only to a 2K partition, with a base offset of zero (i.e. first word of the partition is offset 0000-, last word is offset 3777-).

• The task OVERLAY AREA is 13 sectors long and occupies 0000- to 3177-. This will contain program code, data and work areas. Programs and Overlay Modules are loaded into the area by FETCH subroutine.

• The task MASTER BUFFER is one sector (128 words) long and occupies 3200- to 3377-. Used by FETCH, REWRITE and OVERWRITE subroutines as a one sector disc transfer buffer.

• The task SPOOL BUFFER is one sector (128 words) long and occupies 3400- to 3577-. Used by the SPOOL, UNSPOOL, POST, SPOOL & POST subroutines. A spooling task creates spool records here; an unspooling task receives spool records here (3577- contains the number of the next spool record in the chain). This buffer may be used as workspace (or as an extension to the Master Buffer) when spooling is not required.

• I/O STATIONS only: The task INPUT BUFFER is 128 characters long and starts at 3600-. Used by the GET and SPLIT subroutines and contains the most recent input from the station (ASCII format terminated by NUL byte).

• PRINTERS only: The task INNER PRINT BUFFER is 132 characters long and starts at 3600-. Used by the PRINT LINE subroutine, which always space-fills the buffer. The buffer holds one line of ASCII printout; a map of print positions against offsets is provided.

• The task CONTROL AREA occupies 3704- to 3777-. This is reserved for system use. The current program name (four ASCII characters) is held at 3716-.

Note Task Control Programs and Outer Print Buffers are within the system core space, not part of the task partition.

Programs And Overlay Modules

All Applications programs are held on disc as Overlay Modules. An overlay Module may be from 1 to 13 (decimal) sectors in length, and is identified by an Octal Number, range 001 to 400 (Module numbers 001 to 040 are reserved for Systems programs).

Currently, all overlay modules are contained on disc in the Overlay Module Library, assigned file identifier 02. An index to this library is also held on disc, stating the length of each module and the offset within the-task partition at which it is to be loaded.

A task may fetch overlay modules into its own core partition at any time (see FETCH Overlay Module subroutine), but in practice only the most complex of applications will overrun the 13 sectors available to the initial overlay. On the contrary, it may be found convenient and practical to group several related programs into the same overlay module.

Programs within LOS are identified by four character name (space filled if necessary) and type (I/O Station or Printer). A directory is held on disc giving, against each I/O station program name, the corresponding Module Number and logical Entry Point within the module. A similar directory exists for printer programs.

Overlay Modules are loaded and amended by the utility program OP. A systems utility program (PLM) is available to allocate disc space to a new overlay module, and to maintain the library index and program directories.

When an overlay module is fetched into core by program name, an option of the system may automatically resolve a block of offset addresses placed at the beginning of the module. (The library index contains flags indicating the modules to which the option applies.)

Direct Access

All tasks may transfer data to and from disc held files. Programs refer to files by a two-digit octal number, the File Identifier. Identifiers 00 to 10 are reserved for Systems Files, 11 to 77 for Applications Files.

The association between File Identifier and Data-Set on disc is, in principle, flexible. But as yet LOS does not provide file opening and closing facilities; therefore all files should be regarded as pre-opened by the system. This should be adequate where the machine is dedicated to the application.

Records within a Direct Access data set are identified by their relative record number; the first record number one. Records may be buffered several per sector (128 words), but records exceeding one sector cannot be contained within the task’s Master Buffer, which is the implied transfer buffer for the FETCH, REWRITE and OVERWRITE subroutines. The system provides automatic de-buffering and re-buffering facilities.

Disc Updates

The possibility of a simultaneous update of the same record (or, more precisely, the same sector) by two different tasks is eliminated within LOS because no mechanism is provided to write a record to disc without having first read that record and locked the entire disc transfer system. The lock must still be in force when the record is rewritten or overwritten.

Where it is necessary to prevent updating an entire file for a period of time this must be recognised and dealt with by the programs concerned.

Extension of File Identifier to eight bits

Bulletin No 23 (May 1978)

The FETCH, GET (Fetch Mode) and EXTRACT from FILE CONTROL BLOCK routines have been modified to handle file identifiers 000 to 377 octal.

This increases the number of files accessible from a single file table from 64 to 256. However, the reason for making this increase is to allow related files to be grouped together. System designers are requested to adhere to the following standards when allocating file identifiers to Indexed Files:

Implementation.

The file table remains 64 words long. Existing entries are unchanged. Step xx contains the address of a five-word “Hybrid” File Control Block, format as follows:

The Bootstrap

To initiate LOS proceed as follows.

1. Place any exchangeable disc (MASTER or SECURITY) holding a copy of the Operating System and holding the configuration Table appropriate to the installation onto Device 70 Drive 0.

2. Insert key, turn to MANUAL.

3. Press RESET. Wait for +15 volt light to come on.

4. Load the five-word bootstrap at 000100:

004400

010670

010570

011470

020104

Note:     This bootstrap is permanently core resident whilst LOS is running.

5. Load MA and PC with 000100

6. CLEAR THE SWITCHES.

7. When the disc is READY, press CONTINUE. Turn key to NORMAL and remove.

Initiation takes approximately one second. Provided the disc holding the System Control Record and the disc holding the program library are on-line, all I/O stations will bleep and display PROGRAM?; otherwise all I/O stations will flash a message indicating which disc is required.

Recovery From Security

The above Bootstrap procedure should be carried out up to step 6 inclusive. The appropriate SECURITY disc must be placed on DEVICE 70 DRIVE 0; in the case of recovery of both FIXED and EXCHANGEABLE MASTER Discs this is the EXCHANGEABLE MASTER’s Security Disc.

After clearing the switches at step 6, raise switches 9 and 10. When the disc is READY, press CONTINUE and remove the key. Initiation will occur and copying proceeds automatically. The I/O stations will flash any requests for discs not on line. When copying is complete, all I/O stations will bleep and display PROGRAM?

Additional Facilities Of The Bootstrap

The control panel switch register acts as a function indicator to the initiator:

Bulletin No 8 (February 1978)

LOS may now be bootstrapped off DEVICE 70 FIXED. Method:

• The first word of the bootstrap (at 000100) must be amended to read 004402.

This amendment must be made each time the system is to be bootstrapped off the fixed disc.

Bulletin No 9 (April 1978)

LOS is available for the Molecular Mk I processor with D800 Discs. All applications software developed on Series III/1 and IV/1 is fully compatible except where MULTIPLE SHIFT or MULTIPLE ROTATE register instructions are used.

The bootstrap for the MK I processor is five words at 000100:

000017

000005

004400

010570

011470

020105

To start-up:

1. Insert key, turn to MANUAL and raise STOP. If green RUN light fails to go out, power OFF and power ON.

2. Raise switch 7 only (i.e. all other numbered switches down).

3. Press START FROM SWITCHES.

4. When UNIT 70 is READY, press INSTRUCTION STEP.

5. CLEAR THE SWITCHES (or set required switch register mask).

6. Press CONTINUE, remove key.

System Halts

The system may halt a task (by calling the HALT subroutine) for the reasons given below.

Processor Halts & Loops

LOS Disc Layout

Configuration Table

Purpose:
To remove from the operating system, as far as practicable, installation and hardware dependent parameters. Thus the bootstrap will load the "raw" OS into core, then the initiator will read the 3 sector Configuration Table into 0/0400 and, by reference to the information in the table, convert the OS into a working version. The Configuration Table is then overlaid by the permanently core resident System Control Record.

Notes:

1. The initiator copies 0/0600 to 0/0677 of the table to 0/1200 to 0/1277. This is intended to serve as File Table 0.

2. The initiator copies 0/0700 to number of words stated in 0/0415 of the table to the address stated in 0/0414. This is intended to serve as an area for holding File Control Blocks. In particular the system FCBs should be placed here (i.e. Files 01 - 06 and 10).

3. Elements in the File Table must point to the absolute core address of Word 0 of the corresponding File Control Block at its final position, not to its position in the Configuration Table.

4. The initiator will FETCH into core (from File Table 0 Overlay Library) those overlays nominated in the Resident Overlay Table (pointed to by 0/0402). This table is of indefinite length - it is ended by a zero word. This facility is intended to allow users to write permanently core resident subroutines, etc, available to all tasks, without having to incorporate them into the raw operating system. Three sectors beginning at 3/0000 are available for this purpose.

Amendment Of Configuration Table

1. Bootstrap as usual from 000100 but leave switch 7 up. One of the I/O stations will bleep and display OP?.

2. Enter 1R to read the table into 0/0400.

3. Amend the table as required, and escape back to OP?.

4. Enter 1W to write the table to disc.

WARNING
The above amendment sequence updates the table held on the EXCHANGEABLE cartridge, Device 70, Drive 0 only. Where the fixed cartridge acts as a Master Disc, the amended configuration table must be copied to the Fixed cartridge (by restarting system and using program COPY; at NO OF SECTORS prompt, enter CT). DO NOT FORGET to carry out this step (otherwise security copies of the Fixed Cartridge will not hold the correct configuration table).

Note:
This warning also applies to amendments to the Operating System. To copy the OS, at NO OF SECTORS prompt, enter OS.

Offset Addressing

The Molecular hardware implementation allows a program to run in any page of core (1K words beginning on a 1K boundary) provided it does not require indirect addressing (i.e. does not need to refer to locations outside that page or Zero page).

Indirect addresses must, however, be absolute addresses. Therefore, if a program requiring indirect addressing is to be run in any page without modification:

• Many (but not necessarily all) of its indirect addresses must be specified as offsets from a specified base point (normally the start of the first page of the program).

• These offsets must be resolved to absolute by software before access by Memory Reference Instructions.

LOS assigns each task a fixed 2K-core partition for its exclusive use. When programming it is convenient to specify addresses within the 2K partition as offsets from the beginning of the partition, address references outside the partition being specified as absolute.

An offset address is indicated on coding sheets by a "-" suffix (range 0000- to 3777-), and in core by Bit 17 set (range 200000 to 203777)

All the applications subroutines within LOS allow address parameters to be specified as Offset from the Partition Base or as Absolute, according as bit 17 of the parameter is or is not set, respectively. The RESOLVE OFFSET, RESOLVE OFFSET BLOCK, LOAD A FROM OFFSET, STORE A AT OFFSET subroutines are available to handle offset addresses within user programs. If offset pointers are blocked at the beginning of an Overlay Module an option of the system may resolve the block automatically when the Overlay is fetched from disc by program name (16 words reserved for this purpose should be ample).

Byte Addressing

1 Byte = 8 bits. A Molecular 17-bit word may hold 2 bytes; bits 16-9 are referred to as the Top byte, bits 8-1 as the Bottom byte.

ASCII characters are always held in bytes, one character per byte. Only 7 bits are required to define an ASCII character; the top bit of the byte is not used and therefore always zero in core. Bit 17 of a word containing ASCII is redundant and therefore always zero.

When passing character string addresses to subroutines, the parameter must indicate whether the string begins in the top or bottom byte. The convention is to set bit 16 of the parameter if the string starts in the bottom byte.

Notes

1. Byte addressing imposes a 32K-word addressing limit on the string addressed. This may be overcome (subject to modification of LOS) provided all byte addresses over 32K words are specified as offsets.

2. When processing character strings, it is expedient to convert the byte address to an “absolute byte number”. Example: to set up a certain print line comprising several 11 character numeric fields, it is desired to program a Binary to ASCII conversion loop. This loop involves incrementing P3 of the Convert Binary to ASCII subroutine call by 11 characters each time, and is best achieved as follows:

Core Map

Minimum core requirement for LOS is 12K words; this includes two 2K task partitions (at 5/0000 and 10/0000).

Absolute Zero Page is directly addressable from all tasks and therefore contains common data and pointers, as mapped in the System Reference.